Computerized tomography is a non-invasive diagnostic tool used in neuroradiology and other disciplines to supplement two-dimensional whole body radiography.
Simply stated, tomography requires that a detector, not x-ray film, pick up the radiation emanated from two "slices" of tissue per revolution. Conventional radiology teaches slices that are up to 1.3 centimeters thick. A total of 25-30,000 or more detector readings may be obtained from each slice. These readings are stored digitally and manipulated by a computer that has been programmed to solve a large number of simultaneous equations.
The underlying principle of computerized tomography is that the slight differences in radiation absorption coefficient between tissues of various densities permit differentiation of various organs and body parts. It also permits detection of abnormal densities. A computer, which is an integral part in any tomography display system, provides a digital printout of calculated density values for each "slice". This information is then transformed to a density image which is viewed on a modulated cathode ray tube. The operator of conventional computerized tomography systems can selectively display various density levels in ultra-contrast to enhance visualization.
At the present time computerized tomography is used primarily in medicine. This non-invasive technique now appears to be the simplest and most accurate screening procedure for suspected inter-cranial pathological conditions.
Present computerized tomography equipment is made almost exclusively for medical use. In medicine, the patient, which is the object being examined, consists largely of low atomic number elements. Low atomic number elements require use of low energy x-ray photons.
There is a great deal of industrial interest in using computerized tomography for creating images of metal parts and the like. The advantage is obvious, i.e. the technique is non-evasive and hence non-destructive. Unfortunately, present state of the art medical tomographic equipment is virtually useless for industrial tomography.
Present medical instruments are not suitable for use with samples that contain significant amounts of high atomic number elements. The reason current tomographic detectors are not effective for industrial imaging is because they are not designed to respond to the high energy photons which must be used to penetrate a sample containing high atomic number elements.
The x-ray detector array is a vital component of any tomographic system. Generally prior art detectors comprise a linear array of small narrow detectors. The width of the active areas of each detector element determines the pixel size of the tomograph and thus its resolution.
Depending on the field size and the detector width, as many as a thousand separate detectors can be contained in a conventional tomographic scanning array. Each detector taught by the prior art must be electrically insulated from its neighbors and requires a separate amplifier. This creates a very serious problem in conventional computerized tomography because each of the detector/amplifier combinations must have identical sensitivity and spectral response.
Any change in sensitivity or spectral response between the plurality of amplifiers used by conventional systems produces noise and non-linearity in the output signal corresponding to differences in zero and span for the electric waveform put out by the amplifier.
Two types of detectors are primarily used in prior art computerized tomographic medical instruments. The first is a gas ionization chamber which contains a gas such as xenon at high pressure, i.e. from 5-25 atmospheres. The gas chamber is subdivided into a large number of narrow subdetectors by metal electrodes. Because they use a gas, which has much lower density than a solid, such ionization chambers are completely impractical for use with the high energy x-rays that would be required by industrial tomography to penetrate high atomic number materials, such as steel, lead, titanium, or the like.
The second type of detector uses a solid scintillator such as NaI to convert x-rays into visible light. This light is subsequently measured by a photo multiplier tube. This type of scintillator detector has excellent absorption coefficients and acceptable efficiency. Unfortunately, photo multipliers are bulky, costly, shortlived and very noisy.
It is a purpose of the present invention to provide a small, simple and solid-state device capable of combining the advantages of a solid scintillator with the low-noise characteristics of solid-state electronics.
It is a further purpose of the present invention to provide a very high resolution tomographic detector capable of effectively sensing the high energy x-rays required by industrial tomography.
Another purpose of the present invention is to provide an apparatus that requires only a single, low noise, high gain stabilized amplifier, thus avoiding the zero drift and nonlinear span problems associated with prior art tomographic systems.
Yet another purpose of the present invention is to provide an industrial tomographic detector whose constituent detector segments are capable of being switched to an amplifier by a scanning beam of photons or any particles capable of creating charge carriers in a photoconductor.
Yet a further purpose of the present invention is to provide a simple, solid-state detector that is capable of providing high resolution for industrial tomography at low-cost.